A construction of such a motor is described in U.S. Pat. No. 5,970,600. The motor comprises a frame in which the stator, the rotor assembly and bearings for rotatably journaling the rotor assembly are contained. The stator comprises a stator iron core and windings and it defines an interior space into which the motor assembly can be inserted. In U.S. Pat. No. 5,970,600, the bearings for the rotor assembly are integrated in end caps of the motor frame whereby a compact structure can be obtained altogether.
A problem arising in the manufacture of such an internal-rotor electric motor is that when the rotor assembly is inserted into the interior space of the stator, ferro-magnetic particles may be abraded from the rotor magnet and may enter the working air gap which is essentially defined by the outer contour of the rotor magnet and the inner contour of the stator. It is possible and common practice to protect the entire motor assembly against penetration of foreign particles and soils by providing a housing after the assembly, like in U.S. Pat. No. 5,970,600, but during the assembly of the various motor components the interior of the motor, and in particular the working air gap, is not protected against the penetration of such foreign particles.
Moreover, internal-rotor electric motors according to the prior art have the disadvantage of a relative time-consuming assembly, because it is mainly a sequential assembly in which the stator and at least one bearing must be successively pre-assembled in the housing and then the rotor assembly must be fitted into the stator and bearing. In general, the required concentricity of the components is produced only by a cover-like flange in which also the second bearing is located in which the rotor shaft is rotatably journaled. This assembling step is made much more difficult by the magnetic forces which come from the rotor magnet and are radially and axially directed by interaction with the stator iron core so that a contactless concentric fitting of the rotor is not possible at all or is possible only with major engineering.
From German patent No. 32 37 196, a synchronous micro motor is known which comprises a one-piece pot-like housing which encloses as a ferromagnetically effective yoke the ironless field windings and includes a rotor arranged within the field windings. The permanent magnets of the rotor are arranged on a sleeve made of a magnetically conductive material. The rotor is housed in a hermetically sealed housing containing the bearings and the torque is delivered through a permanent-magnetic coupling one part of which being formed by the permanent magnets of the rotor and the other part of which consists of a permanent-magnetic arrangement on a shaft outside the rotor frame. The hermetic encapsulation of the rotor ensures that no foreign particles can penetrate into the rotor; the bearings are self-lubricated and are protected against environmental impacts. The whole of the motor is assembled by inserting the hermetically encapsulated rotor into the cylindrical hollow space of a pot-like sleeve open on one side which is made of a plastic in which sleeve the ironless field windings are embedded. This sleeve is located in a casing which is also formed like a pot and is made of a ferromagnetic material.
The German patent DE 32 37 196 is already an improvement on the prior art described above, as the assembly of the motor is simplified, the abrasion of ferromagnetic particles from the rotor assembly during the assembly being eliminated from the start due to the ironless field windings which are in addition completely embedded in the plastic.
The arrangement of DE 32 37 196 has the disadvantage that motors having ironless field windings always run with a very low efficiency because of the large air gap. Therefore, they are mainly used as micro motors for high speeds, preferably in the field of dentistry. However, for the delivery of larger torques, as is required for motor vehicle applications, for example, they are completely unsuited.
Moreover, the internal-rotor motor of DE 32 37 196 has the disadvantage that the hermetic encapsulation of the rotor assembly does not allow a direct, mechanical coupling of a load, but that the torque transmission from the rotor to a shaft must be only in an indirect manner, e.g. by means of magnetic coupling, as is described in the patent specification.
U.S. Pat. No. 4,999,533 discloses a motor having an enclosed rotor unit which is inserted into the stator core by press fit. GB-A-2,186,635 discloses a centrifugal pump having a rotor enclosed in a cylindrical component, the cylindrical component being inserted into a stator core. A similar arrangement is known from U.S. Pat. No. 3,733,504. This document additionally discloses that the sleeve of the rotor assembly can be fixed in the stator by means of cement. To align the cylindrical component, the stator core comprises stop edges.